Recently, significant progress has been made in understanding tauopathies and how tau aggregates lead to neurodegeneration by the first description of the atomic structures of tau filaments from Alzheimer?s disease (AD) brain. The atomic models for the core of paired helical filaments (PHFs) and straight filaments (SFs) purified from the brain of an individual with AD were determined by cryo-electron microscopy (cryo-EM) through an ongoing collaborative effort between our laboratory at Indiana University and the MRC Laboratory of Molecular Biology. The studies proposed in this MPI application are a logical continuation of this groundbreaking work. These studies are in response to RFA-NS-18-015 ?Structural Biology of Alzheimer's Disease Related Dementias (ADRDs) Proteinopathies? (U01). Our proposal has 3 specific aims. The first is to generate a cryo-EM map and determine the corresponding atomic models of tau filaments from the brain of individuals with sporadic and hereditary 3R and 4R tauopathies, in a collaborative effort between the Vidal and Ghetti laboratories at Indiana University and the Jiang laboratory at Purdue University. Importantly, our efforts will have the full support of our collaborators at MRC, Drs. Goedert and Scheres. Second, we will generate a cryo-EM map and determine the corresponding atomic models of tau filaments from the brain of individuals with a history of repetitive brain trauma. Lastly, in collaboration with Dr. Nilsson, a leader in the field of amyloid ligands, we will synthesize, identify and characterize 3R and 4R tau-specific ligands. These ligands will be validated using brain tissues of individuals with 3R and 4R tauopathies and working with the Jiang lab we will attempt to determine the corresponding binding site by cryo-EM.